Silicon is widely used in making photovoltaic solar cells, which convert light into electricity. Silicon commonly exists in nature in the form of silica or silicon dioxide (SiO.sub.2). Quartzite, which is composed largely of silica, is widely used as the starting material for obtaining the silicon used in making solar cells. Silicon can be produced from quartzite through well-known processes like the submerged arc process. In the submerged arc process, a carbon-based reducing agent is reacted with the quartzite to produce silicon along with by-products. The silicon produced by the submerged arc process and other processes typically includes impurities like boron, phosphorus, iron, calcium and aluminum.
Silicon can be classified according to its level of purity. Low grade silicon like metallurgical grade silicon is less pure than a higher grade silicon. It is well known that solar cells made from a high grade silicon can be significantly more efficient than solar cells made from a lower grade silicon. Consequently, high grades of silicon are preferred for use in making solar cells. Low grade silicon like metallurgical grade silicon should, therefore, be purified before it is used as solar cell material. The following table compares approximate levels of significant impurities in metallurgical grade silicon and solar grade silicon, a higher grade silicon that can be used for producing efficient solar cells.
______________________________________ Metallurgical Solar Grade Type Grade Silicon Silicon Impurity (parts per million) (parts per million) ______________________________________ Boron 10-20 less than 1 Phosphorus 10-20 less than 1 Iron 800-3000 less than 10 Aluminum 800-3000 less than 10 Calcium 20-1000 less than 10 ______________________________________
Unfortunately, purification of metallurgical grade silicon by conventional methods is difficult and expensive. Thus, a need has arisen for a method of purifying silicon that is effective and inexpensive relative to conventional methods. An object of this invention is to provide such a method for converting low grade silicon into a higher grade silicon that can be used for making efficient solar cells.
"Low grade" silicon is defined as silicon having impurity levels greater than the impurity levels permitted in "solar grade" silicon.